Measurement of the torque applied to a rotating shaft has long been of considerable industrial interest. In particular, a reliable, accurate, and inexpensive torque sensor is crucial to the development of electric power steering systems for automotive vehicles. The specifications of U.S. Pat. Nos. 4,760,745, 4,882,936, 4,896,544, 5,351,555 and 5,465,627, issued to Ivan J. Garshelis, describe previous methods and materials for measuring torque.
The '555 patent and its divisional case, the '627 patent, describe an arrangement for a torque sensor (see FIG. 1 of either patent) aimed for use in an automotive power steering system. It is a non-contact device comprising a ring of magnetic, magnetoelastic material mounted to the shaft whose torque is to be measured. While Garshelis uses the term "magnetoelastic" to properly characterize the effect of elastic torque strain on the magnetization of his torque sensor, this specification uses the term "magnetostrictive" as appropriate to characterize the material that senses the applied torque.
A vital feature of the sensor is confinement of the ring's magnetization to the circumferential direction when no torque is applied to the shaft. When torque is applied to the shaft and conveyed to the ring, the magnetization of the ring tilts away from the circumferential direction, producing an axial component of the magnetization whose magnitude and sign depend on the size and rotational sense of the torque. The axial magnetization generates a magnetic field external to the ring which is measured by a secondary detector, i.e., a magnetic field sensor. In the embodiment of U.S. Pat. No. 5,465,627, the magnetostrictive ring is formed of nickel maraging steel and is mechanically attached to the shaft by a vigorous force fit. Besides providing for attachment, the large hoop stress .sigma..sub.h thus created in the ring plays the crucial role of generating an effective uniaxial anisotropy K.sub.u which keeps the ring magnetization in the circumferential direction in the absence of torque. If the magnetostriction of the ring is assumed to be isotropic, K.sub.u is equal to 3.lambda..sigma..sub.h /2, where .lambda. is the isotropic magnetostriction constant of the ring material. For maraging steel .lambda..about.30 ppm (cf. I. J. Garshelis, "A Torque Transducer Utilizing a Circularly Polarized Ring", IEEE Trans. Magn. 28, 2202 (1992), hereinafter, Garshelis Paper), a relatively small value, and .sigma..sub.h must be on the order of the yield strength of the ring (655 MPa for T-250 maraging steel) to furnish sufficiently large K.sub.u.
This combination has several major disadvantages, including the following. First, generating large and uniform .sigma..sub.h requires large press forces to mate carefully machined matching tapers on the shaft outside diameter and the ring inside diameter. Unless precisely done, this attachment procedure can easily generate axial or radial (i.e., non-hoop) stresses which can cause substantial non-uniformity of the ring magnetization in the absence of torque. Second, with .sigma..sub.h near the yield strength of maraging steel, the coercivity in the circumferential direction is only .about.5 Oe (FIG. 5 of Garshelis Paper), making the output of the device very susceptible to degradation by small stray magnetic fields or residual stresses. Any disturbance to the circumferential magnetization in the untorqued state can result in loss of sensitivity and/or generation of a spurious torque signal. Third, the axial magnetization with torque applied is proportional to .lambda. according to the model of the Garshelis Paper, and the small value of .lambda. makes for correspondingly small output signal.